A sun angle sensor, or simply sun sensor, is an opto-electronic device used to detect the direction from which sunlight arrives. It has always and continues to be a commonly used navigational or attitude control device for both NASA and commercial spacecraft. It can also be used, for example, on robotic rovers or other payloads performing Lunar and Martian surface exploration for sun-position based navigation and for solar attitude monitoring to help maintain robot and/or payload safety during operations.
A conventional, single-axis sun sensor has a pair of adjacent photodiode detectors behind a slit where the edge of adjacency of the detectors is somewhat inclined to the direction of the slit. When the sensor is pointed directly at the sun, the same amount of light falls on each photo detector and their analog outputs are matched. When the direction of sunlight is off-axis in azimuth only, the detector outputs are imbalanced in a relative sense indicative of the direction and amount of azimuthal offset.
Meanwhile, directional offset in elevation only also produces signal imbalance albeit at a rate which is less than in azimuth. The imbalance from a single such sensor is therefore ambiguous with regard to size and direction of off-axis direction of sunlight. Following a substantial amount of calibration, processing signals from two orthogonally disposed, conventional single-axis sun sensors resolves the mutual ambiguities in azimuth and elevation signals and permits correct, if not particularly sensitive or stable, determinations of the sun angles. Some sun sensor models have performance as good as +/−64 deg field of regard at 0.25 deg resolution and +/−32 deg field of regard at 0.1 deg resolution, limited in part by electronic drifts in the photodiodes' amplifier circuitry.
Aside from the technical demerits of poor resolution, un-axiality, axial crosstalk from orthogonal axes, large size, and high power and cost of conventional sun sensors, there are only one or two US vendors of sun sensors, and the going rate for one single-axis device is about $300 K. Multiple single-axis sensors deployed precisely orthogonally are required on each face of a spacecraft to determine sun angle in both azimuth and elevation.
Additional sensors are sometimes required to determine spacecraft roll angle with respect to the sun-spacecraft vector. Typical deployments use two or three sensors on at least three spacecraft faces. Each sensor package comprises a sensor head about the size of a Post-it note pad and an accompanying electronics box about the size of a brick.
Thus it would be advantageous to have a sun sensor that has higher sensitivity and no drift.